This invention relates to an electric meter, and more particularly to a movable coil type meter using an internal magnet.
The movable coil type meter, particularly the internal magnet type meter, since its construction is very simple, is widely used in, for example, a stereophonic appliance as a compact meter for use as an indicator. in this internal magnet type meter, a movable coil is turnably or pivotally supported at a position which is near the N and S poles of an internal magnet provided in the meter and where the magnetic flux density is high. When a current to be measured flows in said movable coil, this movable coil is turned through a prescribed angle due to the magnetic action between a magnetic flux produced by this measurement current and a magnetic flux produced from the internal magnet. The turned angle in this case is proportional to the ampere turns of the movable coil and the magnetic flux density of the internal magnet. The magnetic flux density of the internal magnet is the highest at the N and S poles thereof and, as the distances from both poles become large, is decreased. Now, when the turned angle of the movable coil from both poles of the internal magnet is .theta., the density of that magnetic flux of the internal magnet which the movable coil cuts can be generally said to be proportional to cos .theta.. Accordingly, in such internal magnet type meter, its scale can not be made linear.
In order that the above-mentioned internal magnet type meter can have a linear scale, for example, a magnet wherein magnetic pole pieces made of soft-iron are attached respectively to the N and S poles of the internal magnet so as to obtain a uniform field is employed. In this method, however, the N and S poles must be worked, which, however, is followed by difficulties and requires a considerably high working precision. Accordingly, this meter is not satisfactory for use as a compact meter from the standpoint of mechanical work and cost. Further, even the use of such method causes the magnetic flux density to be gradually decreased with each of the N and S poles as a center, failing to obtain a completely uniform magnetic field.